The present invention relates to improved lost circulation materials that can be used in reservoir and non-reservoir zones of a subterranean formation.
A drilling fluid or mud is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit and providing a hydrostatic head to maintain the integrity of the wellbore walls, and prevent well blowouts. Specific drilling fluid systems, which can be oil-based or aqueous-based, are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation.
Oil-based muds are normally used to drill swelling or sloughing shales, salt, gypsum, anhydrite or other evaporite formations; hydrogen sulfide-containing formations; and high temperature (e.g., greater than about 300° F.) holes, but may be used in other holes penetrating a subterranean formation as well. Oil-based muds are commonly used as treatment fluids for drilling, stimulation, sand control, and completion operations. As used herein, the term “treatment,” or “treating,” refers to any subterranean operation that uses a fluid in conjunction with a desired function and/or for a desired purpose. The term “treatment,” or “treating,” does not imply any particular action by the fluid. On other cases, aqueous-based muds may be used, or emulsion muds having an aqueous component and an oil-based component.
Lost circulation is a common occurrence in drilling operations. In particular, the fluids may enter the subterranean formation via depleted zones, zones of relatively low pressure, lost circulation zones having naturally occurring fractures, weak zones having fracture gradients exceeded by the hydrostatic pressure of the drilling fluid, and so forth. Lost circulation may be a result of treatment fluid being lost to voids within the wellbore and/or the subterranean formation. As a result, the service provided by such fluid is more difficult to achieve. For example, a drilling fluid may be lost to the formation, resulting in the circulation of the fluid in the wellbore being too low to allow for further drilling of the wellbore. In addition, loss of fluids, such as oil-based muds may be quite expensive. Furthermore, the drilling operations may need to be interrupted until the circulation loss problem is solved, which may result in expensive idle rig time. Therefore, a treatment fluid for lost circulation control may be used. By way of nonlimiting example, voids may include pores, vugs, fissures, cracks, and fractures that may be natural or man-made. Several methods may be available for lost circulation control including bridging fractures, providing fluid loss control, sealing surfaces for fluid diversion, or plugging voids. In each method to control lost circulation, the rheological properties of the treatment fluid may be important to the efficacy of treatment. Lost circulation control fluids contain additives that at least partially plug voids, e.g., pores, cracks, or fractures, in a zone causing loss of circulation. These additives, when used in drilling fluids are typically called lost circulation materials. The more general term for controlling the loss of the treatment fluid from any subterranean fluid is “fluid loss control additives.” That is, lost circulation materials are a subset of fluid loss control additives.
Many conventional fluid loss control additives permanently reduce the permeability of a subterranean formation, negatively affect the rheology of the treatment fluid in which they are used, and/or reduce the rate at which the fluid is allowed to penetrate or leak off into desirable locations within the subterranean formation. Moreover, while it may be desirable to control or prevent fluid loss for a given period of time, in some instances it may become desirable to later allow flow through that portion of the formation, such as in a reservoir zone once drilling is complete. Thus, costly and time-consuming operations may be required to reverse the effects of conventional fluid loss control additives on the treatment fluid and/or to restore permeability to those portions of the subterranean formation affected by the fluid loss control additives.
In addition, some known materials used as fluid control loss additives may give rise to other problems. In some instances, the fluid loss control additives used are toxic and thus may harm the environment; this problem may be aggravated because many are poorly degradable or nondegradable within the environment. Due to environmental regulations, costly procedures often must be followed to dispose of the treatment fluids containing such compounds, ensuring that they do not contact the marine environment and groundwater. Thus, it is desirable to use low environmental impact additives for treatment fluids. It would also be desirable to reduce the number of additives needed in a treatment fluid. In addition, some known materials require hydrocarbon treatments to remove them from the formation after they are placed or require a high temperature for removal, or a large volume of under-saturated liquid (such as for the removal of salts) to be removed.
Thus, while many lost circulation materials are known, there is still a need for materials that are non-hazardous, environmentally friendly, and provide the desired control, and are readily available.